OK, turned out to be IC1. Once changed I noted that both LED1&2 start to illuminate as volume and break-up occur, I'm thinking this is what must be clipping the signal to get the distortion - I've seen it done with normal diodes in a fuzz box circuit before.

OK, turned out to be IC1. Once changed I noted that both LED1&2 start to illuminate as volume and break-up occur, I'm thinking this is what must be clipping the signal to get the distortion - I've seen it done with normal diodes in a fuzz box circuit before.

Anyhow, job done. Thanks for the input folks.

Good.

That's right.

Logged

If you say theory and practice don't agree you haven't applied enough theory.

I have had a similar problem with my Marshall Master Reverb 30 5203 and thanks to this thread I fixed it quickly. All the posts above are much appreciated.

On my amp, as I turned up the preamp volume, it was fine up to about 3, then started to fiz and go quiet at 4. The problem was one of the little IC op amps, in my case IC1, an MC1458, easily and cheaply obtainable from ebay or Farnell. It was only a few minutes work to replace it, although desoldering the old one was a little tricky.

For future ref, I noticed that while the amp was faulty, turning up the pre-amp lit only one of the LEDs; after fixing it, they both light up.

All the best and thanks again to the original poster and answerers.Matt

I have had this problem twice. The amp is pretty old (circa 1986), I don’t know what the underlying cause was, but each time I fixed it by replacing either IC1 or IC2. The part number can be found in tiny lettering on the IC and people sell them on ebay for just a couple of pounds. (It was a bit gutting to discover that this classic cranked Marshall sound is all coming from a dirt cheap Texas Insturments IC (insert emoji of choice here)).

The tricky bit is desolding the IC because it has 6 or 8 legs which all need removing. My method is to pull on the top of the IC with a pair of pliers, while giving the all the legs a quick blast with a cook’s blow torch. It’s obviously risky and I have slightly scorched the circuit board, but I was quick enough not to do too much damage and it worked.

I was debating, if having to do all this work, maybe soldering in IC Holders... less chance of damaging the IC with the soldering gun, and in a case like yours, if needed done again, it's a simple pull one out, pop a new one in.

Just curious, had to clean them and the Bass was seized pretty tight, turned but with effort... Deoxit'd them all, no longer dry, and after 6 flushes the Bass is normal again. When opened it up saw they all said Alpha, which I knew made compatible pots for Marshalls, but the originals I suspect were replaced? Didn't originally say Alpha did they?

Part of me wants to replace those while I got it apart. Shame but I don't think Bourns fit. If I recall, the leads are staggered a bit differently on Marshall pots.

Nephew replied finally, pre-existing issue... I'm outta the dog house for breaking his amp. For the price, I guess there are three CA1458's and one TL701... I'll buy them all, maybe replace them all and add the holders. For the price it saves me some time... though always the risk of damage.

Mouser has MC1458P. That is what I would use.Jazz, I think some Marshall circuits get unstable when 1458's are replaced by 4558, that's why it is suggested to stay with the original number in SS Marshall's.